1. Field of the Invention
This invention relates to improved multilayered combustion system components, such as combustor liners or transition ducts of a gas turbine engine, wherein the inner surface comprises a protective thermal barrier coating (TBC), which includes a ceramic top coat and a metallic bond coat, and the outer surface consists of a structural layer bonded to the TBC through the bond coat. The improved qualities of the new components over current components include a superior thermal barrier coating, a better high-temperature structural material, a smoother inside surface, no irregularities (welds) within the component, and excellent reproducibility. This is accomplished by a vacuum plasma spray (VPS) process which is used to form the ceramic top coat layer on a suitable mold, followed by a metallic bond coat layer and ending with a structural superalloy layer. Thereafter, the mold is removed to form the multilayered component of the present invention.
2. Description of the Prior Art
It is accepted practice in the gas turbine industry to provide TBC's consisting of a ceramic top coat and a metallic bond coat (typically an MCrAlY) on the inner surface of preformed combustion system components. Two of the components protected by such coatings are combustor liners and transition ducts, which contain the combustion flame and channel the extremely hot gas (&gt;1,300.degree. C.) to the first stage vanes. The transition ducts in particular have a fairly complex geometry and the presently known technology does not allow for satisfactory coating of internal surfaces of components with such complex geometries.
The current fabrication process of combustion system components, such as combustor liners and transition ducts, consists of: (i) mechanically forming two or more individual sections of the component; (ii) plasma spraying by atmospheric plasma spray (APS) the inner surface of each section to form the thermal barrier coating system; (iii) welding the sections so coated; (iv) plasma spraying by APS the protective TBC coatings on the welds whenever possible; and, for transition ducts, (v) laser drilling cooling holes through the structural wall and the coating. There are several significant problems with components which have been fabricated in this fashion. One problem is the nonhomogeneity at the welds. Weld regions act as weak sites from which failure may initiate due to poor quality finish of both the top coat and the bond coat of the TBC. Also, due to the rough surface of the TBC inherent in the APS process and particularly of the weld regions, an undesirable change in flow pattern of the hot gas is often produced. Moreover, because the current fabricating process consists of mechanically forming sections of the component followed by welding and spraying inner surfaces of these sections, there is a limitation on the choice of suitable superalloys. Only superalloys with high elongation such as, nickel-chromium alloys known under trade names Haynes 230, IN-617, etc. are suitable. Superalloys which do not possess the required elongation or ductility cannot be used with the current fabrication process, even if they possess other superior properties, such as better high temperature strength and creep resistance, e.g. IN-738LC superalloy.
It should be noted that demand on engine performance has increased in recent years for both aero and industrial gas turbine engines. In 1984, the US Air Force created the High Performance Turbine Engine Initiative (HPTEI) in which increasing the combustor and turbine entry temperatures (TET) was a major goal. A similar program known as Advanced Turbine System (ATS) was initiated shortly thereafter by the US Department of Energy (DOE) which envisaged an increase in firing temperatures above 1427.degree. C.
Gas turbine hot-section materials constitute an important limiting factor and are critical to achieving the higher firing temperatures. Current methods of producing closed combustion system components, e.g., combustor liners and transition ducts, to contain and guide the hot gas, have inherent limitations which are difficult to overcome, especially in more demanding conditions, such as higher temperatures and pressures.